The long-term goal of this project is an understanding of the mechanisms that account for CD4 T cell tolerance to antigens that are presented in the secondary lymphoid organs but not the thymus. Work during the last funding period showed that naive T cells that are exposed to antigen in the context of inflammation induced by microbial substances, proliferate and differentiate into memory cells capable of inflammation induced by microbial substances, proliferate and differentiate into memory cells capable of producing IL-2 and anti-microbial lymphokines. Such as IFN- g. In contrast, native T cells that are exposed to antigen in the absence of inflammation, proliferate less well, most of the progeny die, and the survivors enter a hyporesponsive state characterized by poor lymphokine production. In Specific Aim 1, we will test the hypothesis that T cells do not divide enough after exposure to antigen in the absence of inflammation to recover from a transient state of unresponsiveness that occurs shortly after T cell receptor signaling. We will test the idea that the CD28-B7 and OX40-OX40 ligand pathways are key regulators of cell division during the primary response and because of this, regulate unresponsiveness during the secondary response. In Specific Aim 2, the antigen-presenting cell that initiates the tolerance induction process will be identified using in situ methods with the idea that this cell is a dendritic cell. This line of investigation will set the stage for later experiments designed to measure B7 and OX40 ligand levels on these cells under priming or tolerizing conditions. In Specific Aim 3, we will test the hypothesis that the secondary lymphoid tissues are telogenic under uninflamed conditions because T cell division and B7 and OX40 ligand expression on APC is restrained by factors such as TGF-b1 produced by dendritic cells and other phagocytes as they engulf senescent apoptotic cells during normal homeostasis. The level of engulfment of apoptotic material will be manipulated and the effect on tolerance induction will be determined. The aims will be pursued in model systems in which antigen-specific CD4 T cells can be monitored in vivo at all times during the tolerance induction process. The likelihood of success will be strengthened by interactions with other members of the P01 who will also be tracking antigen-specific T cells in vivo in related tolerance model systems.